Gate Valves are used to completely shut off or provide full flow in a pipeline. The valve opens by lifting a barrier in the path of the fluid and allowing unobstructed flow when the gate is fully open. In this article, you will learn about the different types of globe valves, common applications, and repair and maintenance methods.
Gate valves are comprised of a seat, disc, spindles, a gland, and an actuator that controls the valve. The disc moves in and out of the flow path when actuated.
Gate Valve Types
Gate valves can be categorized by the following attributes: disc type, seat construction, stem design, and actuation method.
The disc of a gate valve is entered into a process system and different configurations are specified depending on process requirements. Gate valves can be divided into groups based on the type of disc: double-disc, knife, wedge, and full bore conduit.
Double Disc Gate Valve
The double-disc gate valve consists of parallel seats where the wedge is controlled when the gate is closed. The formation of a tight seal between the seat and disc controls and directs the fluid pressure that is built and forces the disc against the seat.
Knife Gate Valve
The knife gate valve derives from the parallel gate valve. It is not totally enclosed in the body and lifts out of the body upon opening. Knife gate valves are designed to work in harsh environments as they are designed to be corrosion resistant and slice through viscous liquids.
Wedge Type Gate Valve
The wedge type consists of two inclined seats in the shape of a wedge, and they are usually used in high flow or turbulent applications like the steam service. Seating pressure on both sides makes the wedge type gate valve effective for low-pressure applications as well. Wedge material can be solid, flexible or split. Solid wedge is the most commonly used disk by its simplicity and strength. Flexible wedges are casted in one piece and offer strength and flexibility in the setting process. Flexible material also has strong thermal resistance and leak resistance. The split wedge is self-adjusting and aligns with the sides of both seats. Split wedges are suitable for gas and liquid applications that do not condense at normal or high temperatures.
Full Bore Conduit
Full-bore valves seal off the pipe to flow then extend blockage further upstream and downstream parallel to the flow path. This design is to prevent solids from entering the body cavity. This unique design is seen in pipelines that must be scraped or additional protection against debris intrusion is required.
Valve seats may either be metal or resilient. Resilient seats are made of an elastomer and allow free passage of impurities. Impurities can also be absorbed upon gate seating and washed away when the valve is opened. Metal seats may be a better fit for high flow process media or lack compatibility with rubber. Metal embeds impurities in the bore and is unable to flush out like their resilient counterparts. As such, any metal seat eventually loses its ability to be drop-tight. Thermal expansion at high temperatures is also a concern.
Gate valves see two types of stems: rising and non-rising.
Rising stems, also known as the OS and Y type, and are used where it is important to identify if the valve is open or close by immediate inspection.
Non-rising stems feature a stem threaded into the gate. An indicator points at the position of the gate to show whether the gate is open or closed. The stem remains in a vertically stationary position while the gate opens and closes to let the steam or fluids out.
Actuation Method refers to the mechanisms of valve movement and their connection and installation in the main plants. Basic methods for actuation purposes are manual, manual geared, electric, pneumatic, hydraulic, solenoid, or spring. For more information on the type of actuator that a gate valve requires, please review the Punchlist Zero article: Actuators.
Gate valves offer easy installation making for easy maintenance and repair. The pressure drop during operations is small as there is no fixed part in the flow stream. Gate valves are used in a variety of fluid services such as air, fuel gas, feedwater, steam, and oil. They generally compete with balls valves but do not offer the same tight shut-off capability. They do offer some ability to modulate flow, but typically only in a guided disc arrangement for line sizes 6″ and larger.
Gate valves are infrequently used in upstream oil and gas production facilities. The high operating pressures, heavy-duty requirements, and long opening and closing times of gate valves make them a sub-optimal fit for these processes.
Maintenance and Repair
Gate Valves require regular and periodic maintenance for optimum performance. The following general maintenance and repair considerations should be kept in mind:
- Lubrication to prevent aggressive fluids and contamination. This depends on cycle times, debris intrusion, and disc type.
- Cleaning of the gate and seal surfaces.
- The nuts should be checked and tightened at regular intervals.
- The valves should be checked for rust so that further damage can be prevented.